Comparison of the effect of five guest residues on the .beta.-sheet conformation of host (L-val)n oligopeptides

The synthesis, characterization, and IR absorption, 'H NMR, and CD properties of (L-Val)ₙ, host tetra-, penta-, and hexapeptides containing an L-Ile, L-Ala, D-Val, L-Pro, or Aib guest residue are reported, with the intention of establishing a scale of \β-sheet disrupting capability of various amino acids. The results indicate that in the solid state the \β-sheet structures of the (L-Val)ₙ, (n = 5,6) homopeptides are at least partially disrupted by the L-Pro and Aib residues. In CH₂Cl₂/Me₂SO solvent mixtures the following rank order is obtained for the stability of the \β-sheets: L-Val> L-Val > L-Ile \ L-Ala \ D-Val > L-Pro \ Aib. In CDCl₃ solutions the Aib hexapeptide is folded into a partially labile 3₁₀-helical structure. Statistically disordered conformations largely prevail in more polar solvents like 2,2,2-trifluoroethanol and ethanol in all the peptides examined. Only the (L-Val)ₙ, hexapeptide is prone to adopt the \β-sheet conformation upon addition of 40% (v/v) water to the trifluoroethanol solution.